The present invention relates to ultrasound imaging and pertains particularly to the use of scattered ultrasound instead of specular ultrasound reflection for producing an image of the object.
The use of ultrasonic waves for detecting changes in density and the like in optically opaque bodies have been known for some time. Recent developments in ultrasound imaging have resulted in methods and apparatus for diagnostic investigation which has in many instances replaced traditional X-ray techniques. Ultrasound imaging has many advantages over radiography. Among these advantages are the ability to define soft tissues within the body. Other advantages include the lack of noticeable side effects. At energy levels used in medical diagnosis, ultrasound is believed to be completely harmless as opposed to the harmful effects of ionizing radiation which are well-known and accumulate with each additional exposure.
Ultrasound as presently employed utilizes predominantly the type of re-radiation from the interior of the examined object which is termed specular reflection. Specular reflection is a form of radiative return which obeys the law of reflection which states that the angle of incidence equals the angle of reflection.
Specular reflection as utilized in current ultrasound imaging devices exhibits certain specific properties. These include the fact that it follows the laws of reflection and is anisotropic in its propagation, that is, it is directional. Specular reflection is also independent of wavelength and frequency. The signals are typically strong and easily measured.
These are certain drawbacks, however, to the use of specular reflection in gaining information about an object. The main drawback is that, in order to obtain adequate information, it is necessary to scan an object from many different angles and positions. This imposes certain restrictions upon ultrasound in that it takes several minutes to produce a useful image using a perpherally moving scanning technique. It is necessary to either rotate the object or to move the equipment around the object to produce a coherent image. Overlying obstructions to the acoustical path, such as bone or gas, may seriously obscure the desired internal structure or region.
The present invention overcomes most of the drawbacks inherent in the prior art techniques of ultrasound imaging. The present invention is based on the exploitation of a certain aspect of re-radiated sound waves, Rayleigh scattering. Such scattering is re-radiation that does not obey laws of reflection in which the angle of incidence equals the angle of reflection.
When the size of the object is very small in relation to the wavelength of the incident wave, the scattered wave front exhibits several relevant properties. These are: (1) the scattered wave front propagates equally in all directions. This propagation is said to be isotropic. (2) The amplitude of the scattered wave front varies inversely with the fourth power of the wave length of the incident wave. A simplistic expression of this concept can be expressed by the formula a=k(d/L), where "a" is the amplitude of the scattered wave front, "d" is the relevant dimension of the scattering object, "L" equals the wavelength of the incident wave front, and "k" is a constant specific to each experimental arrangement.
It is desirable to provide some method and apparatus that overcomes limitations inherent in current ultrasound imaging techniques. The present invention presents such a method and apparatus.